The invention relates to a resonant converter, a regulation method for a resonant converter and a switched-mode power supply.
A switched converter operating as a DC voltage converter transforms an input side DC voltage into one or more output side DC voltages in that the input-side DC voltage is first chopped i.e. transformed into a switched AC voltage and with this switched AC voltage a resonant arrangement comprising at least one capacitor is fed, which capacitor comprises the primary side of a transformer. On the second side the transformer comprises one or more windings whose voltages are rectified to generate output DC voltages.
Known switched-mode power supplies comprise a power supply input circuit for connection to the electricity power grid, and a switched converter. The power supply input circuit renders an intermediate circuit DC voltage available with which the switched converter is fed. The intermediate circuit DC voltage is transformed into one or more output DC voltages by the converter.
Many circuits for switched converters are known. They are not only resonant converters but also circuits in which no resonant arrangement is used. Switched converters can be instrumental in manufacturing cost-effective small and light power supplies/switched-mode power supplies which may be used to advantage in consumer electronics such as set top boxes, satellite receivers, television sets, computer monitors, video recorders, compact audio sets and so on. For these applications are often necessary converters that generate a plurality of output voltages from one DC input voltage.
Usually, one of the output voltages is regulated to a set value. With state-of-the-art converters which produce a plurality of output voltages, with each of the output voltages being assigned a secondary winding of the transformer, a plurality of output voltages cannot be regulated independently of each other. In such circuits is provided a regulating device for only one of the output voltages. It is assumed then that the other voltagesxe2x80x94which are linked with the one regulated voltage via the ratio of the number of windingsxe2x80x94are xe2x80x9cco-regulatedxe2x80x9d with the latter. However, with very different load on the individual outputs this is highly disadvantageous.
A known topology of a converter comprises the so-termed load resonant converter. In a known circuit for this converter a DC voltage-supplied half bridge is used as an inverter, which half bridge supplies power to a series combination of a resonant capacitor and the primary side of a transformer. The resonant capacitor together with the stray inductance of the transformer as well as further, also secondary-side inductances or capacitors forms a resonant arrangement. On the secondary side the load resonant converter has one or several secondary windings. In this manner a number of output DC voltages is supplied which are customarily filtered by at least one capacitive filter after rectification.
To regulate the output voltage of such a resonant converter it is known to change the drive of the inverter. The switches of the inverter are then controlled so that an AC voltage, in many cases a pulse width modulated voltage is generated having predefined parameters (for example frequency). By variation of the frequency of this voltage the magnitude of the output voltage can be regulated. The output voltage is then increased the closer the frequency comes to the resonant frequency of the resonant arrangement. For LLC converters, operation in the hypercritical area is customary i.e. the resonant arrangement is fed with a voltage whose frequency is higher than the resonant frequencies. During this operation the output voltage can be increased in that the frequency of the voltage is reduced. In known load resonant converters only one output voltage can be regulated directly. Further output voltages are coupled to the directly regulated output voltage via the ratio of the number of windings and are thus xe2x80x9cco-regulatedxe2x80x9d.
The type of converter dominating in consumer electronics is the flyback converter. This is a non-resonant converter. On the primary side the inverter usually needs to have only one switching element. On each of its outputs the flyback converter provides one-way rectification. One of the outputs can be regulated directly. If a second output voltage is necessary in the flyback converter, which output voltage is to be regulated directly, it is known to connect a further converter referred to as step-down converter or buck converter to one of the outputs of the flyback converter, which step-down converter is fed with the output voltage of the first flyback converter and supplies the second output voltage with a separate regulation. Such a circuit comprising two converters is very costly, however.
A further extension of the flyback converter topology which renders two regulated output voltages available is the so-termed double forward flyback converter. A corresponding topology is described, for example, in IEEE-PESC 1988, p. 142: xe2x80x9cA Complete Study of the Double Forwardxe2x80x94Flyback Converterxe2x80x9d by J. Sebastian et al. As with the flyback topology used as a basis here it is not a resonant arrangement, but the primary-side AC voltage that is generated via a simple switch and is directly fed to the primary side of the transformer. On the secondary side there are two secondary units each formed by a secondary winding of the transformer and a one-way rectifier element (diode). The secondary voltages supplied by them are capacitively filtered by one secondary unit and inductively by the other. In this way it is possible to regulate an (inductively filtered) output voltage via the duty cycle of the pulse width modulated voltage and the other (capacitively filtered) output voltage via the frequency of the pulse width modulated voltage. But this xe2x80x9chard switchingxe2x80x9d topology gives rise to considerable switching losses.
In modern consumer electronics appliances it is more and more often necessary to produce a plurality of supply voltages because there are outputs with a lower power consumption and outputs with a higher power consumption
It is an object of the invention to provide a resonant converter, a regulating method and a switched-mode power supply on at least two outputs of which, which outputs are arranged for delivering powers of different magnitudes, predefined voltages can be delivered.
This object is achieved by a resonant converter as claimed in claim 1, a regulating method as claimed in claim 10 and a switched-mode power supply as claimed in claim 11. Dependent claims relate to advantageous embodiments of the invention.
According to the invention a resonant topology is proposed i.e. a resonant arrangement is supplied with power by an inverter which comprises a series capacitor and the primary side of a transformer. Further, secondary-side elements may also form part of the resonant arrangement. In such a resonant topology the secondary voltages can be regulated via the frequency of the primary-side AC voltage. Via hypercritical operation there may be achieved with such a resonant converter that the resonant arrangement acts as an inductive load at the source, so that switching without losses (zero voltage switching) is possible.
According to the invention two types of secondary units are provided which have each at least one secondary winding of the transformer and at least one rectifier element. A first secondary unit (a first type of secondary units, respectively) and a second secondary unit (a second type of secondary units, respectively) are then oppositely oriented. The orientation is understood to mean the winding orientation in connection with the circuitry with the rectifier element. For example, two secondary units of opposite types can be distinguished in that the winding orientation on the shared transformer core is opposite with otherwise the same wiring. It is also possible in case of the same winding orientation of two secondary windings to distinguish the secondary unit of the first and the second type by respectively reversed wiring. Wiring is understood to mean the connection of the rectifier element which is preferably a one-way rectifier element, for example, a diode incorporated in only one branch.
As a result of the distinction between the two oppositely oriented types of secondary units, the two secondary units behave differently after being excited. During operation with an AC voltage the secondary units of the first and second type are successively supplied with power. In the definition used here of the types of secondary units a current flows through the first-type secondary unit in essence during a negative voltage level difference on the primary side of the transformer. During the positive voltage level difference on the primary side of the transformer a current accordingly flows through the second-type secondary unit. As will be explained in detail hereinafter, it is possible to utilize this distinction while respective specific excitation, more or less power is supplied by the first-type or second-type secondary units.
Rectification by the rectifier element generates secondary voltages on the secondary units. They may be used directly as an output voltagexe2x80x94customarily after filtering (preferably capacitive filtering). Such an output for which a secondary unit by itself supplies the output, is referred to here as a xe2x80x9cdirect outputxe2x80x9d. Another form of output referred to here as xe2x80x9cstack outputxe2x80x9d provides that the output voltage is supplied by at least one first-type secondary unit and at least one second-type secondary unit which are connected in series. According to the invention the former output voltage is realized as a direct output by a first-type secondary unit, whereas the second output voltage is realized as a stack output by a first-type secondary unit and a second-type secondary unit. The first output (direct output) is arranged for producing a lower power level and the second output (stack output) for producing a higher power level.
It is then preferred that the ratio of the rated power on the stack output to the rated power on the direct output lies between 1.5:1 and 10:1 if no more than the two former outputs are available. The design should preferably be made so that in nominal operation the two half waves are not loaded very differently. With two outputs and a perfectly symmetrical design, theoretically a 3:1 ratio of the output rated power in favor of the stack output is ideal. Because of the symmetrical design the two half waves are then uniformly loaded so that there is a duty cycle of 50%. Depending on the requirements made to the converter it may be necessary that this converter is capable of permitting either of the two output powers to drop to zero at full power on the respective other output. If the converter has a plurality of outputs, for example, two direct outputs and one stack output, these three outputs may be designed such as regards their rated power that a substantially uniform load of the half waves is achieved.
For the secondary-side circuit of the resonant converter, various output configurations are possible. On the one hand, the first-type secondary unit, which is provided in the form of a direct output for the first output voltage, may be interconnected to the second-type secondary unit in the form of a stack output for the second output voltage. Since each of the two secondary units has only one one-way rectifier element (diode), the total circuit in this case manages with only two such line semiconductors, so that a highly cost-effective structure is possible.
Alternatively, it is possible for the direct output for the first output voltage to be formed by a secondary unit (first-type) and the stack output for the second output voltage by two further secondary units (one first-type secondary unit and one second-type secondary unit). In this case three output rectifiers are necessary. It may then be provided that the direct output is completely electrically isolated from the stack output.
According to a further embodiment of the invention a regulating device is provided for regulating both the first and the second output voltage to a respective set value. For example, the desired output voltage can also be delivered with disturbing magnitudes such as a variable load etc. Such a regulating device takes up respective measured quantities from the resonant converter. In accordance with the regulation method according to the invention the regulating device drives the inverter. The inverter produces a switched AC voltage, preferably a pulse width modulated voltage of usually constant amplitude. To regulate the two output voltages, preferably two correcting variables defining the waveform of the pulse width modulated voltage are used, for example, switching frequency and duty cycle.
Preferably a modulator is used which drives the inverter on the basis of predefined values of the regulating device, in that a pulse signal for driving the switch of the inverter is predefined. Especially with small powers a half bridge is preferred as an inverter for reasons of cost, with which half bridge voltage pulses are generated from an input DC voltage in that two switches mutually switch. Also the use of a full bridge is conceivable.
The idea of regulating device here refers to a functional unit. This functional unit may be realized as an integrated or discrete analog or digital circuit. The regulation, however, may also be completely implemented as a digital control algorithm run on a micro- or signal processor, so that the regulating unit does not of necessity form a separate physical unit of a resonant converter.
A resonant converter according to the invention can produce two individually regulatory output voltages. In many cases, however, consumer electronic appliances need to have a larger number of for example 10 output voltages. If more than two output voltages are necessary for an application, they are divided into two groups where the voltages of each group can be regulated separately from the voltages of the respective other group. The formation of these groups is effected such that secondary units of the first type provide the output voltages of the first group in the form of direct outputs, whereas the second group of output voltages contains such voltages that are fed by secondary units of both the first and second type (stack outputs). Alternatively, second-type secondary units can also supply further output voltages of the second group as direct outputs.
According to a further embodiment of the invention there is provided that the regulating device processes measured values of the first and second output voltage. These two output voltages of which the first one is delivered to a direct output and the second one to a stack output, are measured continuously and the measuring results are applied to the regulating device where they are compared to respective set values.
The regulation of the output voltages irrespective of each other is possible by the utilization of the behavior of the first and second-type secondary units which is different depending on the excitation. When excited by a pulse width modulated voltage, the overall height of the output voltages can be regulated by suitably predefining the frequency (utilization of the increase of resonance). By predefining the duty cycle, the secondary voltages on the secondary units of opposite types can be mutually increased or reduced.
When the first and second output voltages are directly measured and respective regulation differences are formed, a regulation may be effected in that a preset value for the frequency of the switched AC voltage generated by the inverter is calculated from the regulation difference on the output voltage on the stack output. A preset value for the duty cycle of the switched AC voltage is calculated from the regulation difference of the output voltage on the direct output. The calculation of predefined values for frequency and duty cycle from the respective regulation differences is preferably made by known one-dimensional regulators, for example, I, PI, or PID regulators.
In an alternative embodiment the second output voltage produced on the stack output is not measured and regulated directly, but the respective output voltages of the secondary units which feed the stack output are measured and regulated individually. In this case the output voltage on the stack output which in a series combination corresponds to the sum of the individual output voltages on the secondary units is regulated indirectly. This configuration is particularly advantageous when further outputs are needed. Outputs which are fed by first-type secondary units then form a first group and outputs which are fed by second-type secondary units then form a second group. The outputs of the two groups are then coherently regulated within the group but may also be regulated independently of the other group.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.